Yes you can.
When we went a couple of years ago we rented a car in Khorat and drove to Chiang Mai , handing it back there. We booked the car through this website http://carrentals.thaivisa.com/
and the car was rented from Budget. I just had a quick look and they do car hire picking up in Chiang Mai and dropping off in Chiang Khong. We had no problems at all.
#2 john_f has been a member since 26/3/2010. Posts: 18
thank you so much!
#3 franks2k has been a member since 17/1/2011. Posts: 10